Problem: On the first 5 quizzes of his physics class, William got an average score of 77. What does he need on the next quiz to have an overall average of 79?
Solution: Let his score on the next quiz be $x$ The sum of all of his scores is then $5 \cdot 77 + x$ The same sum must also be equal to $6 \cdot 79$ Solve: $x = 6 \cdot 79 - 5 \cdot 77 = 89$.